As shown in FIG. 13, Cable shovels 1 (also known as dippers) are large excavating machines which have been used widely in mining operations for over 150 years. The buckets 3 for these machines may have a capacity as large as 82 cubic yards. The buckets 3 includes an open top 4 for gathering earthen material during digging, and a door 200 forming the bottom wall 5 for dumping the gathered load 325. In regard to cable shovels, the back wall 6 of the bucket 3 is the wall to which the stick 300 (i.e., the end of the boom 301) is attached. The front wall 7 is opposite the back wall 6 and is the wall associated with the lip 302. A pair of opposing sidewalls 8 are each located between the back wall 6 and the front wall 7, and also often supports the ends of the lip.
The doors can be very heavy and at times may weigh over 30,000 pounds. The door has at least one plate with an interior surface to contain the material being excavated. The door 200 is hinged to supports on the back wall so as to be closed during digging, and opened during dumping. A releasable latch is provided to hold the door 200 in the closed position. The size and weight of the door combined with the weight of the load within the cavity of the bucket causes the door to experience stress. Often the load within the bucket is not centered and causes the door to experience twisting, bending, and torsional loading. The load may become off center during loading or may become off center when the operator contacts the tracks of the cable shovel or the edges of the truck body when maneuvering the bucket. The stresses can cause the door to crack and become damaged. To strengthen the door and minimize the twisting, bending, and torsional loading experienced, the plate is generally reinforced with supports. The supports increase the door stiffness and help the door resist the off center loading. The supports, however, increase the weight of the bucket.
In use, the bucket with the door in the closed and latched position is driven into the ground to collect a load. Once filled, the bucket is lifted and the machine rotated to position the bucket over an empty truck body of a mining dump truck. The latch is released to dump the load into the truck body. The door is free swinging so that when the latch is released the potential energy of the weight of the door and the load within the bucket transitions into kinetic energy and causes the door to swing quickly and forcefully downward. The door, when released, can sometimes strike the truck body resulting in damage to the truck and/or the door. To compensate, the operator will often raise the bucket higher than necessary to avoid contact with the truck or will lower the bucket onto the truck floor or the material already in the truck before releasing the door for a more controlled opening of the door. This practice of lowering the bucket also requires an additional lifting of the bucket up out of the truck body with enough lift for the open door to clear the sides of the truck body. This extra lifting and/or lowering on every digging cycle slows the process and results in less production. A higher dumped load can also cause damage to the truck as the load dropped from a higher position impacts the truck body with more force. After the load is dumped, the operator swings the bucket away from the truck and lowers the bucket back to engage the ground. As the bucket is lowered to engage the ground the front wall approaches a horizontal position and the door approaches a vertical position, which causes the door to close under its own weight. The door will at times slam shut with great force and may cause damage to the bucket or the door. The latch, then, automatically actuates to secure the door for another digging pass.
The uncontrolled swing of the door can cause damage to mining equipment and can be dangerous to equipment operators and maintenance personnel. Many devices with varying degrees of success have been used to minimize the damage of the forceful swing of the door. For example, numerous snubbers, such as the snubber disclosed in U.S. Pat. No. 5,613,308, have been used to slow the uncontrolled swing of the door. In addition, hydraulic circuits as disclosed in U.S. Pat. No. 6,219,946, and brake and clutch devices as disclosed in U.S. Pat. No. 6,467,202 have been used to control the swing of the door. Bumpers have been used to protect the bucket as the door closes and to protect the tracks of the cable shovel if the door is opened to close to the cable shovel. The devices used to restrain and dissipate the kinetic energy of the load and the door are expensive, increase the weight of the bucket, and require intensive maintenance.
A typical latch includes several components that add considerable weight to the bucket. Each cable shovel is designed to lift a maximum weight, which is the combined weight of the bucket and the load of excavated material. Accordingly, the more weight that exists in the bucket, the smaller the load that can be gathered into the bucket or the less wear material that can be fitted to the bucket. Moreover, on account of the placement of the latch along the bottom of the door, it is common for these parts to need frequent repair or replacement because of the heavy loads and abrasive materials encountered. The latch system must be regularly adjusted to ensure that the latch fully engages the latch keeper as the latch experiences wear. Adjusting the latch bar because of wear can be very time consuming (i.e., increased downtime which translates into a decrease in productivity).